Recently, as an electronic device has been made more miniaturized, lightened, slimmed, and portable with the development of the information communication industry, a battery used as an electric source for such an electronic device has been strongly required to have a high energy density. Accordingly, a lithium-ion battery, which is the most satisfying battery for such requirement, has been actively researched.
A lithium-ion battery generally has a drive voltage of 3.6V or more (which is higher than approximately three times that of a Ni—Cd battery or a Ni-MH battery), high energy density and an excellent lifetime characteristic, and thus the market related to the lithium-ion battery has rapidly grown. At present, the lithium-ion battery is widely used for portable electronic devices, etc., such as mobile phones, notebook computers, digital cameras, camcorders, etc., and also research on application of a high capacity lithium-ion battery to electric vehicles, hybrid electric vehicles, robotics, aerospace, etc. is actively being conducted.
The lithium-ion battery comprises a cathode comprising a metal oxide containing lithium, an anode comprising a carbonaceous material capable of lithium intercalation/deintercalation, an electrolyte providing a movement path of lithium ions, and a separator interrupting short circuit in the cathode and the anode, and generates electrical energy by oxidation/reduction reactions through intercalation/deintercalation of the lithium ions at the cathode and the anode. Such a lithium-ion battery requires an electrolyte having an electrochemically improved composition so as to achieve a high drive voltage, a low self-discharge rate, high energy density, long cycle life, etc. As such an electrolyte, a non-aqueous mixed solvent including a combination of carbonate-based solvents, such as PC (Propylene Carbonate), EC (Ethylene Carbonate), DEC (Diethyl Carbonate), DMC (Dimethyl Carbonate), EMC (Ethylmethyl Carbonate), is mainly used.
The properties of an organic electrolyte are indicated by main indicators, such as conductivity, electrochemical stability window, operating temperature range, density and stability. In particular, indicators related to the conductivity include solubility, degree of dissociation, permittivity, viscosity, etc. Each of the solvents used as an electrolyte in a lithium-ion battery has its own advantageous/disadvantageous properties, and, thus, battery performance is largely dependent on a combination of such properties of the employed solvents. As described above, a lithium-ion battery mainly uses a non-aqueous organic electrolyte. The non-aqueous electrolyte is widely used despite low conductivity because its electrochemical stability window is wider than that of water, and, thus, it is possible to achieve high voltage of a battery.
In such a lithium-ion battery, one of the biggest problems is low stability. During a formation reaction of a solid electrolyte interface (SEI) film, gas is generated within the battery by decomposition of a carbonate based organic solvent. The generated gas within the battery causes sudden reactions, such as expansion of battery thickness upon charge, decomposition of the organic solvent with the passage of time during high-temperature storage upon overcharge or after charge, etc. Such decomposition of an organic solvent causes stability degradation, such as battery performance deterioration, and fire and explosion of a battery.
Since trimethyl phosphate (TMP) and triethyl phosphate (TEP) (flame retardants for plastic) have been initially applied to a lithium-ion battery, research on a flame retardant, such as tributhyl phosphate (TBP), and hexamethoxycyclotriphosphazene (HMPN), has been continuously conducted. However, there is a problem in that such a flame retardant provides a flame retardant effect while causing battery performance deterioration, such as reduction of charge/discharge cycle life, by ion conductivity of an electrolyte, reversible deterioration of a battery, etc.
In order to solve such a problem, technology of additioning a certain compound to an electrolyte has been developed.
For example, Korean Patent Publication No. 10-0693288 disclosed a method of improving the stability of a battery by adding a mixture of naphtoyl chloride, divinyl adipate, and ethoxy ethyl phosphate, thus suppressing overcharge of the battery. Also Korean Patent Publication No. 10-0585947 disclosed a method of improving battery performance at high C-rate by adding a mixture of trimethylsilyl borate and trimethylsilyl phosphate.
However, in the case of an electrolyte including the above mentioned additives, some effects, such as overcharge suppression or performance improvement at high C-rate, have been known, but other effects of battery performance have not been specifically known. Also when a certain compound is added in an electrolyte in order to improve battery performance, some areas of the battery performance are expected to be improved while other areas may be degraded.
Therefore, an electrolyte, which is not adversely affecting battery performance and at the same time can show a flame retardant effect, has been required to be developed.